The present invention relates generally to the field of demodulation and detection of analog signals carrying digital data, and more particularly to demodulation of an analog signal that is modulated by an odd constellation of signal sample points.
To transmit digital data over the public switched telephone network (PSTN), modulation is necessary to code the digital information into analog signals. A common modulation technique used in digital communications is known as carrierless amplitude and phase (CAP) modulation. Specifically, CAP modulates the digital information into two orthogonal signal components, referred to as the in-phase component and the quadrature component. Thus, digital information can be represented in CAP by a signal comprising a combination of the in-phase and quadrature components.
The signal set representing the digital information is commonly displayed in a two dimensional, signal space or constellation diagram in which the number of points in the constellation is given by 2.sup.n where n is the number of bits to be encoded. Constellations are termed even or odd based on whether n is an even or odd number. Thus, a 16 point constellation is considered an even constellation while an 8 point constellation is considered an odd constellation. Furthermore, as used herein, a 16 point constellation is considered one constellation level higher than an 8 point constellation.
Based on the noise and attenuation characteristics of the communication channel, an optimal modulation profile can be determined. That is, the number of bits per baud and the baud rate will vary according to the transmission characteristics of the communication channel for optimum throughput. In practice, it is not uncommon for systems to be designed to alternate between 8, 16, 32, 64, 128 and 256 point constellations. Even constellations, however, enjoy a geometric benefit in demodulation because they are arranged in a "square" in which all of the individual points fall along a grid allowing a decision device to easily extract the error and recover the original digital information.
Prior art attempts to demodulate odd constellations have often involved translating the individual odd constellation points onto an even constellation decision grid where the error can be extracted and the digital information recovered using even constellation demodulation methods. While these approaches are effective in demodulating odd constellations, the translation and error extraction must be performed at the baud rate, which requires additional processor real time to perform the signal processing.
Accordingly, there exists a need for a system to demodulate odd constellations that can take advantage of the geometrical benefit inherent in even constellations, is not dependent on the baud rate and minimizes any additional processor real time required for signal processing.